1. Field of the Invention
The present invention relates to an optical disk drive, and more particularly, relates to a rotation control circuit, a semiconductor integrated circuit, and a method for controlling the same.
2. Description of the Related Art
Recently, with acceleration of optical disk drive speeds, the technology of rotating an optical disk at high-speed close to the limit of a disk motor and a pickup has become widely developed. An optical disk drive for a disk such as digital versatile disk (DVD) having two layers requires a number of precise high-speed servo controls of the optical disk such as a focus servo and a tracking servo, in addition to controlling the rotation. For example, a focus servo moves the lens in a pickup in the focus direction (direction of optical axis of the lens) based on a focus error signal. By the tracking servo control, track-jump from one particular track to other tracks can be achieved. By the focus servo control, focus-jump from one layer to another layer can be achieved.
An optical disk is fixed to a disk motor by equipment called a clamper. However, the damper cannot fix the optical disk in an ideal condition, because the optical disk has physical dimensional errors. Therefore, an eccentricity may result that means a horizontal shift occurs when the optical disk rotates, and a disk deflection may result that means related error in a shift from perpendicular plane of rotating shaft occurs. That is to say, a relative speed condition between the pickup and the optical disk changes.
Constant linear velocity (CLV) and constant angular velocity (CAV) control are known as main rotational speed control systems. In the case of CAV control, a rotational speed of a CD-ROM at 48× speed drive will reach 8640 rpm at 3 (Hz/speed)×48×speed=144 Hz at the circumference of the disk. In the case of a DVD at 16× speed drive, the rotational speed reaches 9600 rpm at 10 (Hz/speed)×16×speed=160 Hz at the circumference of the disk. During the high-speed rotation, speed of a beam spot on the optical disk also becomes very high. The beam spot shifts over the 68 tracks, when the DVD rotates at 16× speed with a track pitch is 0.74 μm and eccentricity quantity of 50 μm. Therefore, maximum relative speed between the lens and the track reaches 40 KHz, converted into a tracking error signal frequency. The control frequency band of a tracking servo generally reaches 5–7 KHz. Therefore, it is not possible to return to a normal operation when the tracking servo deviates at the maximum speed. A vibration of a pickup mechanism created by the eccentricity of the optical disk also considerably increases.
The relative speed between the optical disk and the pickup lens becomes high when rotational speed of the optical disk becomes very high. The servo system loses the controllable frequency band when seek speed between the optical disk and the pickup lens becomes high. As a result, an open loop gain of the servo system decreases in proportion to an increase in the rotational frequency of the optical disk. As a result, residual error showing the servo performance increases. When the residual error increases, a tracking characteristic of the pickup for the eccentricity and the disk deflection deteriorate. Therefore, a radio frequency (RF) signal that is the information signal deteriorates, and the pickup cannot read the data. As a result, it is difficult for the pickup to accurately executes a jump operation of track jump and layer jump. Seek performance of the optical disk drive also remarkably decreases, when the success probability of the track jump and the layer jump decreases. The seek performance decides the performance for reading the data from the optical disk. In addition, jump failure during high-speed rotation of the disk causes a time loss. Since DVD has only two layers, when the pickup fails to layer jump in one layer of the DVD, the pickup cannot settle in another layer. The control must be designed so that the success probability of track jump and layer jump may be improved by the tracking servo and focus servo, since the servo gain can not be sufficiently ensured and such design is very difficult.